1. Field of the Invention
The present invention relates to a cathode ray tube in which a deflection yoke is installed, and more particularly relates to a cathode ray tube capable of effectively reducing the deflection power.
2. Description of Related Art
An example of a conventional cathode ray tube will be described with reference to FIG. 12. FIG. 12 is a cross-sectional view of a cathode ray tube 20 according to a conventional example. A vacuum envelope 21 comprises a glass panel 22 whose display component is substantially rectangular, a glass funnel 23 whose large-diameter portion is linked to this panel 22, and a cylindrical, glass neck component 25 that is linked to a cone component 24 of this funnel 23.
A fluorescent screen 26 formed from a layer of fluorescent material is provided on the inner surface of the panel 22. This fluorescent layer comprises a striped or dotted three-color fluorescent layer for emitting red, green, and blue light. A shadow mask 27 is disposed across from the fluorescent screen 26. Numerous electron beam passage holes are formed in the shadow mask 27. An electron gun 28 that emits three electron beams is provided inside the neck component 25.
A deflection yoke 29 is installed from the outside of the cone component 24 of the funnel 23 to the outside of the neck component 25. The three electron beams are deflected by horizontal and vertical deflection magnetic fields generated by the deflection yoke 29, then are scanned through the shadow mask 27 horizontally and vertically over the fluorescent screen 26, which results in the display of a color image.
One type of cathode ray tube that is often put to practical use is a self-converging inline type of cathode ray tube. With this cathode ray tube, the electron gun 28 has an inline configuration and emits three electron beams that are disposed inline on the same horizontal plane. The horizontal deflection magnetic field generated by the deflection yoke 29 is pincusion-shaped, the vertical deflection magnetic field is barrel-shaped, and the three inline electron beams are deflected by these horizontal and vertical deflection magnetic fields, so that there is no need for a special correction system, and the three inline electron beams can be converged over the entire screen surface.
With a cathode ray tube such as this, the deflection yoke 29 consumed a great deal of electrical power, and lowering the power consumption of the deflection yoke 29 was key to reducing the power consumption of the cathode ray tube. Meanwhile, the anode voltage that ultimately accelerates the electron beams must be raised in order to increase the brightness of the screen. Also, the deflection frequency has to be raised in order to accommodate HD (high definition) TV or personal computers and other such office automation equipment. All of this results in greater deflection power.
In general, deflection power is reduced by decreasing the diameter of the neck component 25 of the cathode ray tube 20, and decreasing the outside diameter of the cone component 24 where the deflection yoke 29 is installed, so that deflection magnetic field operates more efficiently with respect to the electron beams. In this case, the electron beams pass in close proximity to the inner surface of the cone component 24 where the deflection yoke 29 is installed.
Accordingly, when the diameter of the neck component 25 or the outside diameter of the cone component 24 is further reduced, a phenomenon called BNS (beam neck shadow) occurs. This is a phenomenon in which an electron beam deflected at the maximum deflection angle toward one of the diagonal corners of the fluorescent screen 26 collides with the inner wall of the cone component 24, and part of the electron beam fails to reach the fluorescent screen 26 because of the shadow of the inner wall of the funnel 23 (hereinafter this phenomenon will be referred to as “beam neck shadow”).
JP S48-34349B proposes a technique for solving this problem, in which the cone component 24 where the deflection yoke 29 is installed has a shape that progressively changes from being circular to being substantially rectangular in the panel 22 direction from the neck component 25 side. This arose from the idea that when a rectangular raster is drawn on the fluorescent screen 26, the region through which the electron beams pass on the inside of the cone component 24 is also substantially rectangular.
When the cone component 24 where the deflection yoke 29 is installed is formed in a pyramidal shape, the inside diameter of the diagonal corners where an electron beam is likely to collide (near the diagonal axis: near the D axis) is increased with respect to the ordinary circular shape, so as to avoid electron beam collisions. Deflection power can also be reduced by decreasing the inside diameters in the horizontal axis (H axis) and vertical axis (V axis) directions, so that the horizontal and vertical deflection coils of the deflection yoke are closer to the electron beams, allowing the electron beams to be deflected more efficiently.
However, with a cathode ray tube such as this in which the cross sectional shape of the cone component is substantially rectangular, the closer the cross sectional shape of the cone component is to being rectangular, the more the air pressure resistance of the vacuum envelope decreases, and safety is compromised. Therefore, for practical purposes the shape must be suitably rounded, in which case the problem is that there is no longer any reduction in deflection power.
In regard to this problem, in JP H9-320492A, as the external shape, and sometimes the internal shape as well, of the cone component progressively changes from the neck side in the panel direction from being circular to being a non-circular shape having its maximum diameter in a direction other than the first and second axial directions, and in a coordinate system in which the tube axis includes the origin and the first and second axes intersect at right angles, the angle formed by either of the two orthogonally intersecting axes at a position on the maximum diameter varies with the position on the tube axis.
When we let θ be the angle formed by the first axis at a position on the maximum diameter, and N/M be the ratio between the first axial direction and the second axial direction of the fluorescent screen, the shape is such that tan θ≠N/M. Further, the shape is such that tang is closer to 1 than the value of the ratio N/M of the ratio between the first axial direction and the second axial direction of the fluorescent screen.
JP 2000-243317A proposes a technique for improving the magnetic field generation efficiency of a deflection yoke by making the cross sectional shape of the cone component taller than the aspect ratio of the screen in a cathode ray tube in which the cross sectional shape of the cone component is substantially rectangular.
However, the shape discussed in the above-mentioned JP H9-320492A is such that the angle formed by either of the two orthogonally intersecting axes at a position on the maximum diameter varies with the position on the tube axis. Consequently, the diagonal shape of the cone component becomes complex, the glass thickness distribution of the diagonal corners also becomes complex, and it is difficult to ensure adequate air pressure resistance. Also, the angle θ formed by the first axis at a position on the maximum diameter has a wide specified range, and when a shape is attempted such that the value of θ is closer to 1 than N/M, there will also be a region in which deflection power increases, and it is difficult to set the angle θ properly.
According to the construction of JP 2000-243317A, deflection magnetic field efficiency can be improved by making the aspect ratio of the cross sectional shape of the cone component taller than the aspect ratio of the screen. Here, the angle θ formed by the horizontal axis and a position on the maximum diameter of the inner surface of the cone component is not the proper angle at which beam neck shadow can be prevented, so preventing beam neck shadow and reducing deflection power are mutually exclusive. Furthermore, when the cross sectional shape of the cone component is too much taller than the aspect ratio of the screen, this too can lead to an increase in deflection power, so that it is difficult to set the angle θ properly.
An objective of the present invention is to solve these problems encountered in the past, and to provide a cathode ray tube with which air pressure resistance is ensured and beam neck shadow is prevented when the deflection magnetic field of the deflection yoke is closer to the electron beams. This would allow the electron beams to be deflected more efficiently, and reduce deflection power.